Cited below are examples of rotation angle sensors used in a vehicle body control system of a vehicle or the like.
There exist a method and a device for measuring the angle of a rotatable body as disclosed in Patent Document 1, for instance. This is an example of a sensor for detecting rotation angle of a rotatable body, such as an automotive steering wheel, which rotates within a limited but over single-turn range. The device shown in FIG. 12 disclosed in Patent Document 1 detects the rotation angle from angles of rotation of two rotatable bodies 50 and 51 having a phase difference.
Also, as an example, a below-described rotation angle sensor is disclosed in Patent Document 2. Referring to FIG. 13, the rotation angle sensor disclosed in Patent Document 2 is such that two gear portions 259 are attached to a rotary shaft (not shown) of which rotation angle is to be detected via a hooking spring 260. These two gear portions 259 are engaged with gear portions 262 which hold code disks 261 having outer peripheral surfaces on which different magnetic poles are alternately arranged. As the magnetic poles provided on the code disks 261 move in rotary motion as a result of rotation of the rotary shaft of which rotation angle is to be detected, it is possible to detect the rotation angle of the rotary shaft by counting displacements of the magnetic poles by means of magnetism sensing devices 263 which are disposed face to face with the outer peripheral surfaces of the code disks 261.
With this rotation angle sensor attached to two shafts which are interconnected by a torsion bar, for example, it is possible to detect torque by comparing rotation angles of the rotary shafts when the torque acts between the two shafts and torsion occurs between the shafts.
A manufacturing process of the rotation angle sensor of FIG. 13 includes a magnet forming step for magnetizing the magnetic poles of the code disks 261 and a mounting step for mounting the gear portions 262 in such a way that the gear portions 262 mesh with the gear portions 259. Generally, the magnetic poles of the code disks 261 are formed by setting the gear portions 262 to which the unmagnetized code disks 261 have been attached in a magnetizer and magnetizing the code disks 261 so that the different magnetic poles are alternately formed along a circumferential direction of each code disk 261 at specific intervals. The two gear portions 262 are attached to the rotary shaft in such a way that the gear portions 262 face each other.
Also, while a structure for torque detection is generally used in an electric power steering system, known types of electric power steering system include rack-assist type, pinion-assist type and column-assist type which are selected according to properties and specifications of the respective types. The rack-assist type and the pinion-assist type are disposed at a steering gearbox at an axle side whereas the column-assist type is disposed at a steering column. Structures for detecting rotation angle disclosed in Patent Documents 1 and 2 are disposed at a steering column on a steering wheel side in most cases.
The structure disclosed in Patent Document 1, however, used to have a problem in that there has been a possibility that a large measurement error could occur if rotation angles detected by the two rotatable bodies 50, 51 deviate due to gear looseness, for instance.
In the structure disclosed in Patent Document 2, a rotation angle sensing portion for detecting the rotation angle and a torque sensing portion for detecting the torque are combined to form a single unit. Therefore, it is impossible to use the structure disclosed in Patent Document 2 if it is intended to dispose the rotation angle sensing portion at the steering column and the torque sensing portion at the steering gearbox, for instance.                Patent Document 1: Japanese Unexamined Patent Publication No. 1999-500828        Patent Document 2: Japanese Unexamined Patent Publication No. 1999-194007        